This invention relates to a lead frame particularly excellent in reliability at solder joints and a semiconductor device using the same.
In due to the recent increasing demand on surface mount type semiconductor devices leads of these devices are directly soldered onto a substrate. Generally speaking, since a linear thermal expansion coefficient of a semiconductor device (a package) and that of a substrate are different, when subjected to repeated temperature changes, thermal strain takes place repeatedly in solder joints, often resulting in causing solder joint fracture due to thermal fatigue. This is a serious problem from the viewpont of reliability.
In order to improve the reliability of lead solder joints of semiconductor devices, Japanese Patent Unexamined Publication No. 59-36952 discloses a process wherein solder wettability of the whole surface of the lead is bettered, and Japanese Patent Unexamined Publication Nos. 59-108334 and 61-270856 disclose processes wherein the shape of the leads is improved so as to broaden a solder joint area of the leads. According to the process wherein solder wettability of the whole surface of its leads is bettered, a thermal fatigue life of the solder can be improved since concentration of strain due to defects of the solder joint can be prevented. But, according to this process, there is a danger of covering the whole surface of the leads with solder when a larger amount of solder is used. In such a case, since relative displacement of substrate and package due to temperature change cannot be absorbed by bending deformation of the leads, strain in the solder joint is increased reversely. As a result, solder fatigue life often decreases. Particularly when leads become finer and finer as in semiconductor packages of recent years, there is a large possibility to solder in excess. On the other hand, according to the process wherein the joint area of solder is enlarged, the average strain of the solder may be lessened, but there is little effect on improving the thermal fatigue life due to no consideration on portions causing maximum strain.